1. Field of the Invention
The present invention relates to a thermal flow meter for detecting the flow rate of a fluid with a thermosenstive resistance, and more particularly to a thermal flow meter outputting a pulse-like signal corresponding to the flow rate of a fluid, and a fuel controller using the thermal flow meter adapted for, e.g., internal combustion engines.
2. Description of the Related Art
Generally, very high accuracy is required in flow rate conversion that is to be performed on a signal of a thermal flow meter in a fuel controller for, e.g., internal combustion engines. Therefore, a high-accuracy reference power supply is provided in the fuel controller, and A/D conversion is made on a flow rate signal on the basis of the reference power supply.
FIG. 9 is a block diagram showing an example in which a conventional thermal flow meter outputting a voltage signal is employed in a fuel controller for internal combustion engines.
In FIG. 9, a thermal flow meter 1 is connected to a fuel injection controller 2 which comprises a high-accuracy reference power supply 3, an A/D converter 4, and a microcomputer 5. The thermal flow meter 1 includes an amplifier (not shown) which amplifies a flow rate signal applied thereto and outputs an amplified signal.
A voltage signal output from the thermal flow meter 1 is applied to the A/D converter 4 in the fuel injection controller 2, and the applied voltage signal is A/D-converted on the basis of the high-accuracy reference power supply 3. A resulting digital signal is used to carry out a process of fuel injection amount control in the microcomputer 5.
FIG. 10 is a block diagram showing an example in which a conventional thermal flow meter outputting a current signal is employed in a fuel controller for internal combustion engines, the example being disclosed in, e.g., Japanese Unexamined Patent Publication No. 2-216420.
In FIG. 10, a high-accuracy reference resistance 6 is disposed between the input side of an A/D converter 4 and the ground side. The other structure is the same as shown in FIG. 9.
In this example, a current signal output from a thermal flow meter 1 is subjected to current/voltage transformation (I/V transformation) by the high-accuracy reference resistance 6 in a fuel injection controller 2, and then applied to the A/D converter 4. The applied voltage signal is A/D-converted on the basis of a high-accuracy reference power supply 3. A resulting digital signal is used to carry out a process of fuel injection amount control in a microcomputer 5.
If a resistance value of the high-accuracy reference resistance 6 has a temperature characteristic or changes depending on the individuals, a resulting variation directly gives rise to an error of the flow rate. For this reason, the high-accuracy reference resistance 6 is usually formed of a resistor which is expensive and has very high accuracy.
In the conventional thermal flow meters, however, when it is used in combination with the fuel injection controller, the high-accuracy reference power supply is required for the A/D converter in the fuel injection controller to suppress an error that may occur during the A/D conversion. Also, in the thermal flow meter outputting a current signal, a high-accuracy reference resistance, which has a superior temperature characteristic or small variations (in temperature characteristic, resistance value, etc.) depending on the individuals, is required to suppress an error that may occur in the current/voltage transformation. There has been hence a problem that the conventional thermal flow meters are necessarily expensive.